This invention relates to an exposure apparatus and a method of correcting for air pressure. More particularly, the invention is suited to a projection exposure apparatus in which a correction is made for a fluctuation component of an image-formation characteristic that accompanies a fluctuation in atmospheric air pressure or ambient air pressure within the apparatus, thereby making it possible to achieve highly precise projection exposure.
In a projection exposure apparatus used to manufacture a semiconductor device, a circuit pattern that has been formed on a mask or reticle is transferred to a photoresist layer on a wafer or glass plate, which serves as a photosensitive substrate, with a high degree of overlay precision. In order to accomplish this, it is required that the reticle and wafer be positioned (aligned) highly precisely.
Focus calibration is well known as a technique for making the focal point of the wafer agree with that of the reticle.
FIG. 1 is a schematic view illustrating the projection exposure apparatus having a focus calibration function based upon the TTL (Through The Lens) method. The apparatus shown in FIG. 1 includes a light source 1 for exposure. When a circuit pattern on a reticle 2 is transferred to a wafer 8 by exposing the wafer to the pattern, an exposure-apparatus control system 70 transmits a command to a light-source control system 30, and the operation of the light source 1 is controlled by a command from the light-source control system 30.
The reticle 2 is held on a reticle stage 4. A reticle reference plate 3 is held on the reticle stage 4, though there are instances where the plate 3 is secured at a position that is optically equivalent to that of the reticle 2.
In a scanning-type exposure apparatus, the reticle stage 4 is capable of being moved along the optic axis (z) of projection optics 5, and along axes (x, y) perpendicular to this axis, and is also capable of being rotated about the optic axis.
Control to drive the reticle stage 4 is carried out by sending a command from the exposure-apparatus control system 70 to a reticle-stage control system 40 and implementing control in accordance with a command from the reticle-stage control system 40.
Though not shown, several types of reference marks are provided on the reticle reference plate 3.
The projection optical system 5 is constituted by a plurality of lenses. When exposure is carried out, the image of the circuit pattern on the reticle 2 is formed on the wafer 8 at a magnification that corresponds to the reduction magnification of the projection optics 5. A projection optics control system 50 will be described later.
A projection optical system 6 and a detection optical system 7 form an off-axis autofocus optical system. The projection optical system 6 emits a non-exposure light beam that is condensed on a point on a stage reference plate 9 (or on the top of the wafer 8) and is reflected from this point. The reflected light impinges upon the detection optical system 7. Though not shown, a photoreceptor element for position detection is placed within the detection optical system 7 and the arrangement is such that the photoreceptor element and the light reflection point on the stage reference plate 9 will be conjugate points. Accordingly, a deviation in position along the optical axis of the projection optics 5 on the stage reference plate 9 is measured as a deviation in the position of the incident light beam on the position-detection photoreceptor element in the detection optical system 7.
A deviation in position from a predetermined reference plane of the stage reference plate 9 measured by the detection optical system 7 is transmitted to a wafer-stage control system 60. When focus calibration (described below) is measured, the wafer-stage control system 60 performs control to drive the stage reference plate 9 up or down along the optic axis (z direction) of the projection optics 5 in the vicinity of a predetermined reference position. The wafer-stage control system 60 also controls the position of the wafer 8 at the time of exposure.
Described next will be components for sensing the state of focus on the wafer 8 and driving a wafer stage 10 to detect the optimum focal point of the wafer 8 with respect to the reticle 2.
An image detection optical system 20 for focus calibration has elements 21, 22, 23, 24, 25, described later. Illuminating light emitted from a fiber 21 passes through a half-mirror 22 and is condensed in the vicinity of the reticle reference plate 3 (or reticle 2) via an objective lens 23 and mirror 24.
The illuminating light that has been condensed in the vicinity of the reticle reference plate 3 is condensed on the stage reference plate 9 via the projection optics 5. The top of the stage reference plate 9 is provided with reference marks (not shown) of several types. Light reflected from the stage reference plate 9 returns along the original optical path, traverses the projection optics 5, reticle reference plate 3, mirror 24 and objective lens 23 in the order mentioned, is reflected by the half-mirror 22 and impinges upon a position sensor 25.
The stage reference plate 9 is placed on the wafer stage 10 in a manner similar to that of the wafer 8. The stage reference plate 9 is fixed in a focal plane equivalent to that of the wafer 8.
The exposure-apparatus control system 70 manages the focal-point positions on the top surfaces of the wafer 8 and the stage reference plate 9 with respect to the projection optics 5, or the amount of focus offset between both surfaces and the projection optics 5.
The operation of a TTL-based focus calibration will now be described in detail.
FIG. 7 is a flowchart illustrating the sequence of focus calibration. With reference to FIGS. 1 and 7, the detection optical system 20 is focused coarsely on a reference mark on the reticle reference plate 3 (or on a mark on the reticle 2) (step S701). The purpose of step S701 is to focus the image detection optical system 20 on the mark of the reticle reference plate 3 (or reticle 2).
This will be described taking as an example a case in which the stage reference mark is measured while shifting the focal-point position of the stage reference mark at 100-nm intervals over a range of from xe2x88x921439 nm to +361 nm.
First, the stage reference plate 9 is moved to a position at which the reference mark on the stage reference plate 9 can be observed by the image detection optical system 20 (step S702). The focal point of the stage reference mark is xe2x88x921439 nm at step S702.
The procedure represented by steps (1) to (3) below (the loop of steps S703 to S705) is repeated until the focal-point position of the stage reference mark becomes +361 nm. In the repetition process, the value of the quantity of light or the contrast value that prevails when the focal point of the stage reference plate 9 is varied with respect to the projection optics 5 is measured. The measured value of the quantity of light or the measured contrast value is stored in association with the focal point of the stage reference plate 9 prevailing at the time of measurement.
(1) The reference mark is measured by the image detection optical system 20 (step S703).
(2) The focal point on the top surface of the stage reference plate 9 with respect to the projection optics 5 is measured by the autofocus detection system (the projection optical system 6 and detection optical system 7) (step S704). (It should be noted that the order of steps S703 and S704 may be reversed.)
(3) The focal point of the stage reference plate 9 with respect to the projection optics 5 is changed (step S705). More specifically, the stage reference plate is driven +100 nm from its present position.
On the basis of the value of the quantity of light or contrast value thus obtained by the foregoing measurement, an approximation calculation or center-of-gravity calculation is performed to compute the optimum focal-point position of the stage reference plate 9 (or wafer 8) with respect to the reticle reference plate 3 (or reticle 2) (step S706).
A method of correcting for a change in the image-formation characteristic that accompanies a change in air pressure will now be described with reference to FIG. 1.
As shown in FIG. 1, the exposure apparatus is provided with a barometer 80 for reading atmospheric air pressure or ambient air pressure within the apparatus. The pressure value read by the barometer 80 is transmitted to the exposure-apparatus control system 70.
The exposure-apparatus control system 70 calculates the amount of change in air pressure from the pressure value transmitted by the barometer 80 and exercises control so as to correct for the change in the image-formation characteristic if the amount of change in air pressure exceeds a predetermined quantity. There are four examples of methods of correction for dealing with a change in image-formation characteristic, namely (a) driving a movable stage along the optic axis of the projection optics 5, (b) driving a correction lens in the projection optics 5 along the optic axis, (c) changing over the wavelength of the light source, and (d) varying the scanning speed of the reticle stage 4. These four examples will now be described in brief.
In correction method (a) of driving the movable stage along the optic axis of the projection optics 5, the wafer-stage control system 60, which has received a command from the exposure-apparatus control system 70, subjects the wafer stage 10 to a correction by applying a drive command for moving the stage to a position that has been made to reflect an amount of correction for dealing with the change in the image-formation characteristic that accompanies the change in air pressure.
In correction method (b) of driving the correction lens of the projection optics 5 along the optic axis, the projection optics control system 50, which has received a command from the exposure-apparatus control system 70, subjects the correction lens (not shown) to a correction by applying a drive command for moving the lens to a position that has been made to reflect an amount of correction for dealing with the change in the image-formation characteristic that accompanies the change in air pressure.
In correction method (c) of changing over the wavelength of the light source, the light-source control system 30, which has received a command from the exposure-apparatus control system 70, subjects the exposure light source 1 to a correction by applying a drive command so as to change over the wavelength of the light source (not shown) to a wavelength that has been made to reflect an amount of correction for dealing with the change in the image-formation characteristic that accompanies the change in air pressure.
In correction method (d) of varying the scanning speed of the reticle stage 4, the reticle-stage control system 40, which has received a command from the exposure-apparatus control 70, subjects the reticle stage 4 to a correction by applying a scanning drive command that causes an amount of correction, which is for dealing with the change in the image-formation characteristic that accompanies the change in air pressure, to be reflected in a scanning speed conforming to the ratio of the scanning speed of the reticle stage 4 to the scanning speed of the wafer stage 10.
In a correction method of varying the scanning speed of the wafer stage 10, the wafer-stage control system 60, which has received a command from the exposure-apparatus control system 70, subjects the wafer stage 10 to a correction by applying a scanning drive command that causes an amount of correction, which is for dealing with the change in the image-formation characteristic that accompanies the change in air pressure, to be reflected in a scanning speed conforming to the ratio of the scanning speed of the wafer stage 10 to the scanning speed of the reticle stage 4.
In a projection exposure apparatus used to manufacture a semiconductor device, a circuit pattern that has been formed on a mask or reticle is transferred to a photoresist layer on a wafer or glass plate, which serves as a photosensitive substrate, which a high degree of overlay precision. In order to accomplish this, it is required that the reticle and wafer be positioned (aligned) highly precisely.
Further, since progress is being made in reducing the line width of the circuit pattern formed, the focal depth is growing ever smaller.
Further, in the prior art, a correction is not applied with regard to short-term changes in air pressure, such as changes in air pressure that occur during measurement for focus calibration.
However, in view of the shorter focal depth and the need for high alignment precision, it is no longer possible to ignore a fluctuation in image-formation characteristic ascribable to a short-term fluctuation in air pressure.
Furthermore, since flexible manufacturing systems for the production of ASICs or the like are currently in vogue, there is a demand for higher throughput in a projection exposure apparatus. In the prior art, however, the appropriate correcting timing is not decided, nor is the correction method selected, from the point of view of throughput or correction precision. As a consequence, the apparatus is not always subjected to a correction by the optimum pressure correction method.
Accordingly, an object of the present invention is to provide an exposure apparatus and a method of correcting for air pressure in the exposure apparatus, in which a change in image-formation characteristic ascribable to a fluctuation in air pressure is corrected for and a correction method that makes the proper correction in the viewpoint of apparatus throughput or correction precision possible is realized.
According to the present invention, the foregoing object is attained by providing an exposure apparatus comprising: exposure means for repeatedly projecting a reticle pattern onto a substrate, which has been placed on a substrate stage, via projection optics to expose the substrate to the pattern; a focus detection system for detecting focus of the projection optics; a barometer for measuring at least one of atmospheric pressure and ambient air pressure within the apparatus; calibration means for calibrating the focus detection system; and correction means for acquiring a correction quantity, during execution of the calibration, which is for correcting for a change in image-formation characteristic ascribable to a fluctuation in air pressure of the projection optics, based upon a result of the measurement by the barometer, and a correction operation of the calibration means using the correction quantity.
According to another aspect of the present invention, the foregoing object is attained by providing an exposure apparatus comprising: exposure means for repeatedly projecting a pattern of a reticle, which has been placed on a reticle stage, onto a substrate, which has been placed on a substrate stage, via projection optics to expose the substrate to the pattern; a barometer for measuring at least one of atmospheric pressure and ambient air pressure within the apparatus; and correction control means for selecting one of a plurality of correction means for correcting for an amount of change of an image-formation characteristic ascribable to a fluctuation in air pressure, and correcting an exposure operation using a fluctuation in air pressure, which has been obtained as a result of measurement by the barometer, and the correction means that has been selected.
In still another aspect of the present invention, the foregoing object is attained by providing a method of correcting for a change in an image-formation characteristic ascribable to a fluctuation in air pressure in an exposure apparatus having exposure means for repeatedly projecting a reticle pattern onto a substrate, which has been placed on a substrate stage, via projection optics to expose the substrate to the pattern, a focus detection system for detecting focus of the projection optics, and a barometer for measuring at least one of atmospheric pressure and ambient air pressure within the apparatus, the method comprising: a calibration step of calibrating the focus detection system; and a correction step of acquiring a correction quantity, during execution of the calibration, which is for correcting for a change in image-formation characteristic ascribable to a fluctuation in air pressure of the projection optics, based upon a result of the measurement by the barometer, and correcting operation of the calibration step using the correction quantity.
In still another aspect of the present invention, the foregoing object is attained by providing a method of correcting for a change in an image-formation characteristic ascribable to a fluctuation in air pressure in an exposure apparatus having exposure means for repeatedly projecting a pattern of the reticle, which has been placed on a reticle stage, onto a substrate, which has been placed on a substrate stage, via projection optics to expose the substrate to the pattern, and a barometer for measuring at least one of atmospheric pressure and ambient air pressure within the apparatus, the method comprising: a correction control step of selecting one of a plurality of correction processes for correcting for an amount of change of an image-formation characteristic ascribable to a fluctuation in air pressure, and correcting an exposure operation using a fluctuation in air pressure, which has been obtained as a result of measurement by the barometer, and the correction process that has been selected.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.